This invention relates to a lens barrel of a so-called flashmatic lens in which during flash photography a distance ring is coupled to an aperture ring to automatically obtain a correct exposure, and, particularly, to a close-up lens barrel.
In normal flash photography, the relationship of a distance d between a light source and an object, an aperture value F of a photographic lens and a flash guide number G is as follows: EQU G=d.times.F . . . (1)
From equation (1), it is apparent that the correct exposure can be obtained by the distance d and the aperture F which satisfy the equation (1). However, in close-up photography an effective aperture value instead of the aperture F given in equation (1) must be given by: EQU F.times.(1+m) . . . (2)
where: m is the photographic magnification.
From equations (1) and (2): EQU G=d.times.F.times.(1+m) . . . (3)
Thus, in the close-up photography, it is apparent that the correct exposure can be obtained by the distance d and the aperture F which satisfy the equation (3).
If f is a focal distance, .increment. is an amount of advancing, and the distance d between the light source and the object is made equal to the photographic distance, the relation of these parameters can be expressed according to a lens formula as follows: ##EQU1##
By substituting equation (4) into the equation (3), the following equation can be obtained: ##EQU2##
From equation (5), it is apparent that the equation (5) defines a curve with a maximum value at .increment.=f/2 and a point of inflection at .increment.=f, which asymptotically approaches zero.
In the flashmatic lens, the distance ring must be rotated to advance the lens barrel and also rotate the aperture ring in order to satisfy the exposure conditions mentioned above. Accordingly, a cam groove is required for advancing. Particularly, in the case of a normal flashmatic lens having a small amount of advancing, when the distance ring coupled to the aperture ring is rotated through the cam groove, as the amount of advancing .increment. is increased, the aperture value F also increases. This is apparent from equation (5). Thus, the aperture ring can be rotated along the cam groove together with the rotation of the distance ring.
However, in close-up photography requiring a large amount of advancing, as is also apparent from the equation (5), the increase of the aperture value F becomes gradually less. Particularly, as the amount of advancing .increment. increases at .increment.=f/2, the aperture value F decreases. Thus, in the case where the distance ring is coupled to the aperture ring, advancing must be carried out without the rotation of the distance ring only in the vicinity of .increment.=f/2. Accordingly, in order to achieve this, attempts to have the distance ring advance before .increment.=f/2 or a precision differential mechanism has been proposed. However, the former is disadvantageous in that it is necessary to provide a scale for photographic distance (photographic magnification) which displays the bidirectional movements of the distance ring, resulting in a decrease of operability. Also, the latter is disadvantageous in that the mechanism becomes complicated and can not be used in the close-up lens barrel having a large amount of advancing.